With trends toward miniaturization and high-density mounting of electronic components, high density is required for printed circuit boards. For example, an interposer is known as a printed circuit board for packaging by mounting LSI chips. Although bonding between such LSI chips and the interposer has been primarily performed by wirer bonding processes, flip-chip mounting tends to increase. Furthermore, the number of pins in packages is rapidly increasing.
With such trends, holes having small diameters with fine pitches, called via holes, must be provided in the interposer.
Though such drill has been performed by a mechanical machining process using a fine mechanical drill or by an exposure (photo-via) process, laser light is being used recently. A drilling apparatus using laser light is superior to mechanical machining using fine drills in view points of processing rate and ready forming fine holes. As laser light, CO2 laser and harmonic solid-state laser are generally used because of a low price of the laser oscillator and reduced operation costs.
Drill by a known laser drill apparatus is as follows. A laser beam from a laser oscillator is introduced to an optical scanning system having a biaxial galvanometer mirror called an X-Y scanner or galvanoscanner via an optical path including a reflecting mirror and the like. The optical scanning system scans the laser beam and introduces the beam on a printed circuit board through a processing lens (for example, Japanese Unexamined Patent Publication No. 10-58178). Since positions of holes to be formed in the printed circuit board are preliminarily determined, the optical scanning system is controlled according to the information on the positions of these holes to form holes one by one.
Problems to be Solved by the Invention
However, in the one-by-one drill using the optical scanning system having the X-Y scanner or galvanoscanner, the operation time increases in proportion to the number of the holes in the printed circuit board. Since the response of the galvanoscanner is about 500 pps, it is difficult to form 500 or more holes per second. Suppose that holes with a diameter of 50 μm are arranged at a pitch of 0.2 mm in a square package board with a side of 10 mm, the number of the holes is 2,500. In this case, the processing time required is 2,500/500=5 sec at a drill rate of 500 holes/sec.
Accordingly, an object of the present invention is to provide a method for laser drill that can form many holes within a short time compared with known laser drilling methods.
Another object of the present invention is to provide a method for laser drill that can select an appropriate processing pattern for a workpiece.
Still another object of the present invention is to provide an apparatus for laser drill suitable for the above method.